Conducting polymers exhibit electrochromism, the ability to reversibly switch colors when external electronic biases are applied. Owing to their flexibility, low cost and high coloration efficiency, conducting polymers have desirable properties not found in other known electrochromic materials, such as inorganic oxides and small organic molecules. The extended π conjugation along the conducting polymer backbone renders optical absorption which often falls in the visible region. The energy gap between the HOMO and LUMO changes with the external bias, and results in absorption shift and visible color change.
The achievable color is an intrinsic property of a specific conducting polymer material, and is applicable to color-related electrochromic applications. For example, poly(3,4-ethylenedioxythiophene) (PEDOT), shows a signature blue color which is darker in the neutral state and lighter in the oxidized state. Some conducting polymers shows polychromism, which means they may have intermediate color states. By tuning the polymer chemical structure, the electronic character of the π system can be adjusted to give different colors. For example, poly(dimethyl-3,4-propylenedioxythiophene) (PPropOT-Me2) switches between purple and light sky-blue.
Copolymerization of two different monomers is one way to obtain new colors without additional chemical modification. Copolymers with precise composition have been made by chemical polymerization in order to achieve the desired color spectrum. Additionally, copolymerization has been carried out by electrochemistry, however, the resulting copolymers did not necessarily have the same composition as the feeding component ratio. In either chemical or electrochemical polymerization, a laborious process is usually involved: products with different combination and ratios of the monomer components have to be synthesized and characterized in separate batches. In addition to the time consuming process, a large amount of electrolyte solvent, salt and leftover monomers generated from each batch raise environmental concerns.
Electrochromic devices are traditionally fabricated by first depositing electrochromic films on indium-doped tin oxide (ITO) substrates from a monomer solution, and then assembling the film into a device by sandwiching a UV-curable polymer electrolyte between the ITO with electrochromic film and another piece of bare ITO electrode. The polymer electrolyte crosslinks upon UV exposure, changes from liquid to a solid-state transparent gel which holds the two ITO pieces together. This method is not efficient because the film quality is greatly affected by the cleanliness of the substrates, the monomer solution, and the method generates a large amount of waste.
Recently, an in situ electrochromic device (ECD) assembly approach has been developed. In this method, electrochromic monomer is mixed with electrolyte before crosslinking, and polymerization of the electrochromic monomer occurs after the device assembly in the solid-state. This method not only significantly increases the success rate of the device fabrication, but also renders a solid gel matrix inside the device before the polymerization of the electrochromic monomers.
There remains a need in the art for methods to rapidly and efficiently determine the color of electrochromic copolymers to significantly accelerate the color selection process.